The present invention relates to a spacer with a cell-formed lattice-work for retaining parallel elongated elements such as fuel rods into a bundle in the fuel assembly of a nuclear reactor. The spacers are thereby arranged in a suitable spaced relationship (about 5-6 dm) between each other along the bundle, and the cells of the spacers in the proposed device are formed from tubular sleeves, each of which offers a passage for one of the elements mentioned in order to fix the elements in relation to each other.
It is important that the above-mentioned spacers do not form too high a resistance to the coolant flow which flows inside the fuel assembly along the bundle and through the spacers.
Previous attempts to provide improved cooling in a boiling-type nuclear reactor (BWR) are known, wherein a number of sub-spacers are arranged and each sub-spacer takes up only a small part of the whole cross section of the bundle and thus braces the fuel rods only in this cross section. This may lead to problems in bracing the sub-bundles against each other. Another known solution comprises providing the bundle with spacers which alternately fix only the fuel rods positioned peripherally in the bundle (peripheral spacers) and the centrally located fuel rods positioned inside the former fuel rods (central spacers), respectively. Also in this case problems may arise in fixing the sub-bundles thus obtained in relation to one another.
In addition, the lattice-work in the above-mentioned known spacers consist of crossed plate bands standing on edge, which substantially form square cells. In a BWR this type of lattice provides less favourable cooling conditions than a lattice composed of sleeves in that the sleeves are given a circular shape which is able to conform better to the normally round fuel rods.
The invention is characterized in that substantially half the number of sleeves in a spacer, evenly distributed over the cross area of the bundle, are located in a first plane across the bundle and joined together so as to form a lattice-work of sleeves and gaps. The remaining sleeves of the spacer are joined to each other and located in a second plane, separate from the first plane, across the bundle, similarly forming a lattice-work of sleeves and gaps. The sleeves in the different planes are arranged in such a way that an element fixed by a sleeve in the first plane traverses a gap arranged in the second plane, and vice versa.
By means of the invention a spacer is provided in which the material area in the planes separated from each other is reduced to almost half of the material area of a spacer in which all the sleeves are positioned in the same plane across the bundle. Since the material area of the lattice-work in a plane across the bundle is in direct relation to the resistance provided by the lattice-work against the coolant flow, the cooling of the fuel assembly is improved most considerably.
Further, the spacers are provided with first and second side plates. In a boiling-water reactor (BWR) these side plates rest against the wall of the fuel box and the bundle is therefore fixed in a satisfactory manner in the lateral direction. In a pressurized-water reactor (PWR) the side plates of the spacers in one fuel assembly will rest against the side plates in an adjacent fuel assembly and this contributes in a corresponding manner to the fixing of the bundle.
According to a suitable embodiment, the first and second planes of a spacer are joined together by sleeves by joining together the first and second side plates. The lower ends of the sleeves in the first plane may then be positioned at a small distance (about 2-3 mm) from or on a level with the upper ends of the sleeves in the second plane. Alternatively, the sleeves may be joined together such that a certain small overlap arises between the ends of the sleeves.
According to another embodiment, especially adapted to a boiling reactor with an elongated fuel box against the walls of which the spacers rest, the spacer part with the first plane is arranged at approximately half a normal spacer distance from the spacer part with the second plane. The spacer parts are joined together by means of a guide sleeve surrounding the bundle and provided with windows, the guide sleeve having an external shape which provides a close fit against the walls of the fuel box. The guide sleeve provided with windows will then scrape off coolant flow flowing along the walls of the fuel box and throw it against the centre of the fuel assembly. This, in combination with the lower flow resistance of the sub-spacers, results in a very efficient cooling of the fuel rods of the bundle.